Chapter 7

Satellite Remote Sensing in Precision Agriculture

Acquiring Satellite Imagery

Historically, only governments and large corporations have had access to quality satellite images. In recent years, satellite image datasets have become available to anyone with a computer and an internet connection. The quality, quantity, and precision of these datasets are continuously improving, and there are many free and commercial platforms available for acquiring satellite images. On top of that, the prices of acquiring the images have fallen significantly, as well as the prices and availability of the tools that will allow you to view and analyze the images. There are two types of available satellite data. There are public datasets that are freely available and of good quality for many use cases. And there are several commercial outfits that offer even better images with more potential uses. Landsat and Sentinel provide the best-known public datasets. Maxar, Planet Labs, Airbus Defence & Space, Imagesat, and Skywatch primarily provide commercial datasets.

Image Requirements

The first step is to determine what kind of satellite imagery is needed for GIS analysis. Depending on the analysis objectives, high-resolution imagery for detailed mapping or low-resolution imagery for large-scale monitoring may be required. Choosing the appropriate spatial resolution depends largely on the application objective, the agricultural field's size, and the farm equipment's ability to use this data. Spatial resolution refers to the distance a pixel represents in an image. For example, NASA's Landsat collects imagery at 15-meter resolution—so every pixel in one of its images represents a 15 m by 15 m square on the ground. Equally important, and sometimes even more important, is the temporal resolution. Precision agriculture applications such as yield estimation require a spatial resolution of 1 to 3 meters.

Free Satellite Imagery Sources

There are two types of available satellite data: public datasets that are freely available and have quality good enough for many use cases, and several commercial providers that offer even better quality images with more potential uses. Google Earth and Google Maps are two of the most widely used options, as they offer access to high-resolution satellite imagery from various sensors and dates, which can be exported as georeferenced images or KML files.

NASA’s Earthdata

NASA’s Earthdata is the provider of online access to multiple collections of NASA’s EOSDIS (Earth Observing System Data and Information System) satellite imagery. Most of NASA’s Earth Data is an analysis product for the study of Earth Science.

EOSDA LandViewer

EOSDA LandViewer is a GIS database with an easy-to-master interface that gives free access to the most widely used open-data satellite maps, offers free previews of high-resolution satellite imagery, allowing the farmer to select and order only the ones required, and features extensive analytic tools. EOSDA LandViewer offers miscellaneous global satellite imagery. Free satellite images from Landsat 7, Landsat 8, Sentinel-1, Sentinel-2, CBERS-4, MODIS, aerial data from the National Agriculture Imagery Program (NAIP), or historical satellite imagery from Landsat 4 and 5. Unlike most free satellite imagery providers, EOSDA LandViewer enables instant visualization and analysis of the maps you’ve found.

Copernicus Data Space Ecosystem

Copernicus Data Space Ecosystem is one of the biggest repositories of open and free Earth observation data in the world. Copernicus delivers global data from satellites and ground-based, airborne, and seaborne measurement systems. It includes both new and historical Sentinel, Landsat, and SMOS imagery, as well as the Copernicus Contributing Missions data immediately available for downloading, streamlining, and on-demand processing. Users can use all the functionalities of the system free of charge, with some predefined quotas.

USGS Earth Explorer

The United States Geological Survey (USGS) is an excellent source of freely available satellite imagery data. The selection of space-derived imagery in EarthExplorer is overwhelming, from optical and radar data to weather satellite photos to digital elevation maps.

Google Earth and Google Maps

Google Earth and Google Maps used to be very different products, but over the past few years, they’ve become much more similar. The main difference between the two is that Google Maps is accessed through a Web browser and an internet connection. Google Earth, in contrast, has been a program that is downloaded and saved on the computer, though now it does have a web browser plugin functionality.

Sentinel Hub

Sentinel Hub provides free access to extensive open-source satellite imagery data via its two services: EO Browser and Sentinel Playground. EO Browser holds around a dozen medium- and low-resolution free satellite imagery sets, including complete archives of all the Sentinel missions, Landsat 5-8, MODIS, Envisat Meris, Proba-V, and GIBS products.

Commercial Satellite Imagery Sources

Commercial satellite data is increasingly used in agriculture to provide detailed, timely, and scalable insights about crops, soil, weather, and land use. These data come from satellites operated by private companies, often offering higher resolution and faster revisit times than public (governmental) satellites.

Mobile Apps for Accessing Satellite Image Data

Mobile apps are not typically used to directly "acquire" satellite image data in the sense of commanding a satellite to take a picture of a specific location. Satellite imagery is typically acquired by specialized equipment on satellites orbiting Earth.

Image Preprocessing of Satellite Imagery

Once the satellite data is required, several preprocessing steps, such as georeferencing, projection, resampling, mosaicking, clipping, enhancement, and classification, are often required to improve the quality and usability of the imagery. Georeferencing assigns spatial coordinates to the imagery, while projection transforms the imagery from its original coordinate system to a standard or desired coordinate system.

Analysis of Satellite Imagery

The final step is to analyze the satellite imagery using GIS systems. Common methods include change detection, which involves comparing two or more images of the same area or scene from different dates or times to identify the changes that have occurred.

Click on the following topics for more information on satellite remote sensing in precision agriculture.